The present invention relates to membrane compositions such as are useful for the separation of gases and for the separation of liquids, and in particular, to methods for preparing such membranes.
Separation processes such as dialysis, ultrafiltration, and reverse osmosis have been used in the separation of a wide variety of impurities from solutions. The development and improvement of membranes for such systems has permitted their use, for example, in the desalination of brackish and saline waters. Further development of membrane technology has prompted the use of such technology for the separation of gases. Thus, it is possible to use certain membranes for the separation of gases, as for example, helium from methane. In addition, membrane technology involving pervaporation processes, membrane distillation and separation in chlor-alkali cells have been developed.
Common membrane compositions are the cellulose ester membranes which are typically employed in flat sheet or hollow fiber form. The properties of such membranes can be defined in terms of permeation selectivity of components to be separated, permeation flux for one of the components to be separated, and mechanical strength of the membrane composition. The permeation flux is a measure of the rate at which one of the components to be separated permeates through the membrane. Permeation selectivity is an indication of the ratio of the permeation fluxes of the components to be separated. A desirable high permeation flux is typically provided by employing a membrane which is as thin as possible. Unfortunately, such thin membrane compositions typically have poor mechanical strengths and are too weak for practical uses.
Typical processes for preparing such membranes involve providing a microporous structure for support and a dense layer for separation using Loeb-Sourirajan method. Such a method can provide asymmetric membranes which provide many desirable features for component separation. Unfortunately, such a method involves process limitations which are difficult to control, and as noted in U.S. Pat. No. 4,430,807, can often involve the necessity of numerous drying techniques. In addition, the use of such techniques generally involves the requirement that the discriminating layer be of the same material as the support layer. Such a requirement is not always desirable. Furthermore, in the preparation of thin film composites, the adhesion of the discriminating layer to the support can be not as great as would be desirable.
Numerous attempts have been made to improve both the membrane compositions and the processes for their preparation. For example, Henis et al., Science, Vol. 220, pg. 11 (1983) disclose a gas separation membrane composition which comprises a coating of silicone rubber in an attempt to cover flaws present in the membrane. Unfortunately, such membrane compositions exhibit poor permeation selectivities in certain applications. Attempts to provide solvent crazed polystyrene microporous membranes by Michaels et al., Advances in Polymer Science, Vol. 27 (1978) have proved unsuccessful. In particular, such membranes exhibit a small increase in flux and are too weak for practical uses.
Also of considerable interest are membrane films which are useful in numerous packaging applications. For example, it is highly desirable to provide films with high tensile strengths which are thin, and which are permeable to certain gases (e.g., such as carbon dioxide) and impermeable to other gases (e.g., oxygen). It is particularly desirable that for packaging applications, the film be thermoformable, flexible and weatherable.
In view of the deficiencies of the art, it would be highly desirable to provide a membrane composition and a process for preparing same, which membrane composition exhibits good permeation flux and selectivity, good mechanical strength, and which can be prepared in an efficient and effective manner.